Phenoxy resin is one of thermoplastic aromatic polyether resins having a high molecular weight (commonly 20,000 or more), which is grouped as an independent unique resin different from thermosetting epoxy resins having a low molecular weight (commonly 5,000 or less) and has features such that the resin can form films by itself and has a flexibility. This phenoxy resin has so superior adhesion, flexibility and impact resistance that it can be used as an adhesive and a coating material without any assistant action of a curing agent or catalyst ("Plastic Handbook", pages 499-501, published in 1986 by Asakura Shoten).
Meanwhile, as electronic equipment such as liquid-crystal television sets, word processors, personal computers, IC (integrated circuit) cards and electronic pocketbooks are made compact and thin-gage, there is an increasing demand for making IC packages more compact and thin-gage. To meet such a demand, IC chips are encapsulated with a liquid encapsulating material in the field of bare chip packaging such as TCP (tape carrier package), COB (chip on board), COG (chip on glass) and MCM (multi-chip module). TCP making use of such a liquid encapsulating material has already been prevailing in ICs for driving LCD panels. In future, this TCP is expected to create a rapid increase in demand with progress of LCD's and other related products. TCP making use of a film tape in the substrate is greatly characteristic of enabling bend packaging. In such packaging, the encapsulating material undergoes a tension when the tape is bent, and hence the encapsulating material is required to have a flexibility, a toughness and a high adhesion to the film tape.
In encapsulating materials, epoxy resins are widely used, which have overall superior electrical characteristics, heat resistance and adhesion. Cured products thereof, however, commonly have disadvantages that they have a high modulus of elasticity and are hard and brittle, and may cause a decrease in adhesion such as peel strength. For this reason, it has been attempted from old times that epoxy resins are modified by, e.g., blending or pre-reacting them with reactive elastomers to impart flexibility and toughness and also to relax internal stress.
For example, Japan Adhesion Society, Vol. 16, No. 7, pages 271-277 (1980) and Japan Adhesion Society, Vol. 17, No. 5, pages 192-200 (1981) report that a heat-cured product of a blend comprised of a bisphenol epoxy resin (molecular weight: 380; EPIKOTE 828), a polybutadiene having a terminal carboxyl group and a butadiene-acrylonitrile copolymer can be improved in elongation, impact strength and peel strength. Japan Adhesion Society, Vol. 17, No. 12, pages 507-513 (1981) reports that a heat-cured product of a blend comprised of a bisphenol epoxy resin (molecular weight: 380, EPIKOTE 828) and a polybutadiene having a terminal carboxyl group can have a low internal stress. Japanese Patent Publication No. 63-60067 also discloses a liquid IC-encapsulating resin composition comprising a liquid epoxy resin (molecular weight: 380; EPIKOTE 828) blended similarly with from 10 to 40 PHR of a butadiene-acrylonitrile copolymer having a terminal carboxyl group.
As methods for modifying epoxy resins with the polybutadiene elastomer having a terminal carboxyl group, not only the above simple blending but also a method is available in which the epoxy resin is beforehand allowed to pre-react with the polybutadiene elastomer having a terminal carboxyl group. This pre-reaction is reported in detail in The 30th Heat-curable Resin Symposium, pages 13-16 (1980), The 31st Heat-curable Resin Symposium, pages 69-72 (1980), J. Appl. Polym. Sci., Vol. 26, pages 907-919 (1981) and The 22nd Adhesion Research Report Meeting, pages 78-79 (1984). Thus, what has been attempted is chiefly the modification of epoxy resins having a small value of epoxy equivalent weight ["SECCHAKU (Adhesion)", Vol.32, No. 11, pages 16-18 (1988)]. In particular, The 22nd Adhesion Research Report Meeting, pages 78-79 (1984) reports that a cured product of an epoxy resin (EPIKOTE 828; molecular weight: 380; epoxy equivalent weight: 184 to 194) pre-reacted with a butadiene-acrylonitrile copolymer having a terminal carboxyl group can be improved in peel strength and tensile shear strength. Japanese Patent Application Laid-open No. 61-287952 also discloses an IC-encapsulating resin composition comprising a liquid epoxy resin modified (pre-reacted) with a similar butadiene-acrylonitrile copolymer having a terminal carboxyl group, serving as a stress relaxation agent.
As another method for making the cured product have a low internal stress, a method is available which utilizes the relaxation of linear polymers. Japan Adhesion Society, Vol. 22, No. 5, pages 255-261 (1986) reports that the cured product has a low internal stress when a phenoxy resin used as a linear polymer is added to the bisphenol epoxy resin (molecular weight: 380; EPIKOTE 828). Japanese Patent Application Laid-open No. 61-228060 discloses an epoxy resin composition comprised of i) an epoxy resin pre-reacted with a butadiene-acrylonitrile copolymer having a carboxyl group, ii) a phenoxy resin and iii) a curing agent. Japanese Patent Publication No. 7-30284 discloses a cold-drying, baking coating composition comprised of a hydroxyl-group-containing high-molecular-weight resin and a blocked isocyanate. Japanese Patent Application Laid-open No. 2-147618 discloses a resin composition in which a fine powder of a blocked isocyanate and/or dicyandiamide is used as a curing agent for curing liquid epoxy resins and phenoxy resins.
Phenoxy resins can be used as adhesives and coating materials without any assistant action of a curing agent or a catalyst. Conventional phenoxy resins, however, can not ensure by themselves a sufficient adhesion strength (peel strength) to adherends, depending on purposes. Thus, they are sought to be more improved in adhesion strength (peel strength).
Meanwhile, the epoxy resin modified by blending or pre-reacting it with a butadiene-acrylonitrile copolymer having a terminal carboxyl group, disclosed in Japanese Patent Publication No. 63-60067 and Japanese Patent Application Laid-open No. 61-287952, can not attain by itself a sufficient adhesion strength (peel strength) especially to polyimide tapes which are base materials for TCP. Similarly, the epoxy resin modified with a butadiene-acrylonitrile copolymer having a carboxyl group and the phenoxy resin, disclosed in Japanese Patent Application Laid-open No. 61-228060, can not attain by themselves a sufficient adhesion strength (peel strength) to the polyimide tapes.
The composition disclosed in Japanese Patent Publication No. 7-30284 contains no epoxy resin, and hence can not attain a sufficient adhesion strength (peel strength) to the polyimide tapes. Also, when an isocyanate is used in which a compound serving as a curing agent of epoxy resin and being non-volatile is used as a blocking agent, any unreacted blocking agent may remain in the cured product after dissociation of the block isocyanate, because no epoxy resin is contained. This may adversely affect the properties of cured products, such as water resistance, mechanical properties and adhesion, as so predicted. As for the resin composition disclosed in Japanese Patent Application Laid-open No. 2-147618, similarly it can not attain a sufficient adhesion strength (peel strength) to the polyimide tapes. Also, the epoxy resin and the phenoxy resin are blended by heating and melting them after the phenoxy resin is brought into a powdery state, where the resultant composition has a very high viscosity of from 2,000 to 20,000 poises at 50.degree. C. Hence, in a case where an inorganic filler is added, the composition may have so higher a viscosity that it is very difficult to use it in liquid encapsulating material. It is difficult to use such baking coating composition or sealing adhesives as they are, in the field of semiconductors as in TCP liquid encapsulating material. Thus, from a stand point of research and from a technical viewpoint, it is a great subject in various resin compositions to design materials that can satisfy the properties of molding materials for encapsulating electronic components.